Rails and TDD

Do the model first, since the controller will depend on the model.

I did this exact same exercise when I was learning rails and it's a
GREAT way to pick up both TDD and Rails because down the line you'll
be able to compare your results against some of the login generators
out there and see where you and the others differ.

It'll also give you a thorough understanding of your login system
which sadly, if generated, can be a hole in someone's intellectual
command of one of the most crucial pieces of their application.

For learning TDD I suggest checking out Geoffrey Grosenbach's peep cast on it:
http://peepcode.com/products/test-first-development

Oh and also if you want a headstart on the login system Chad Fowler's
Rails Recipes has some nuggets for both authentication and
authorization.

I haven't viewed this particular peepcast but I've watched the others
and they are excellent.

Good luck,

Tim Case
tim@karmacrash.com